Global Constructors and Destructors in C++

Global constructors and destructors in C++ have to be handled very carefully to meet the language specification. Constructors have to be called before the main function. Destructors have to be executed after it returns. Under ELF, this can be treated gracefully by the compiler. For example, the GNU C/C++ compiler, gcc, provides two auxiliary start up files called crtbegin.o and crtend.o, in addition to two normal auxiliary files crti.o and crtn.o. Together with the .ctors and .dtors sections described below, the C++ global constructors and destructors can be executed in the proper order with minimal run-time overhead.

.ctors

This section holds an array of the global constructor function pointers of a program.

.dtors

This section holds an array of the global destructor function pointers of a program.

crtbegin.o

There are four sections:

• The .ctors section. It has a local symbol, __CTOR_LIST__, which is the head of the global constructor function pointer array. This array in crtbegin.o only has one dummy element.

• The .dtors section. It has a local symbol, __DTOR_LIST__, which is the head of the global destructor function pointer array. This array in crtbegin.o only has only one dummy element.

• The .text section. It contains only one function, __do_global_dtors_aux, which goes through __DTOR_LIST__ from the head and calls each destructor function on the list.

• The .fini section. It contains only a call to __do_global_dtors_aux. Please remember it has just a function call without return since the .fini section in crtbegin.o is part of the body of a function.

crtend.o

There are also four sections:

• The .ctors section. It has a local symbol, __CTOR_END__, which is the label for the tail of the global constructor function pointer array.

• The .dtors section. It has a local symbol, __DTOR_END__, which is the label for the tail of the global destructor function pointer array.

• The .text section. It contains only one function, __do_global_ctors_aux, which goes through __CTOR_LIST__ from the tail and calls each constructor function on the list.

• The .init section. It contains only a function call to __do_global_ctors_aux. Please remember it has just a function call without return since the .init section in crtend.o is part of the body of a function.

crti.o

It has only a function label _init in the .init section and a function label _fini in the .fini section.

crtn.o

It has only a return instruction each in the .init and .fini sections.

At compile time while generating the relocatable files, gcc puts each global constructor on __CTOR_LIST__ by putting a pointer to the constructor function in the .ctors section. It also puts each global destructor on __DTOR_LIST__ by putting a pointer to the destructor function in the .dtors section.

At link time, the gcc driver places crtbegin.o immediately before all the relocatable files and crtend.o immediately after all the relocatable files. In addition, crti.o was placed before crtbegin.o and crtn.o was placed after crtend.o.

While generating the executable file, the link editor, ld, concatenates the .ctors sections and the .dtors sections from all the relocatable files to form __CTOR_LIST__ and __DTOR_LIST__, respectively. The .init sections from all the relocatable files form the _init function and the .fini sections form the _fini function.

At run time, the system will execute the _init function before the main function and execute the _fini function after the main function returns.